how to treat mold on surface of potting mix

A solution of 3% hydrogen peroxide and water mixed 1 to 1 in a spray bottle can be used. Spray the surface, it will not harm the plant and after it oxidizes the mold it becomes plain old H2O.

It's not terrible, but you should get rid if it... try putting cinnamon on it; that should kill the mold. You can also store the surface up with a firm or something... It might be that your soil is not graining enough, and holding too much moisture!